Audio teleconferencing is a widely accepted business tool, conducted using conventional telephone equipment. Multi-party telephone calls may be set up both by individual subscribers having the proper equipment and by the various local and long distance telephone companies. The equipment required is simply that necessary to form multi-way connections through the public switched telephone network (PSTN).
Video teleconferencing is a newer form of remote business communication tool, conducted through the PSTN using both conventional telephone equipment and specialized video communication equipment. Video teleconferencing may be conducted over conventional analog telephone lines or over newer digital or integrated services digital network (ISDN) telephone lines. Generally, the apparatus for performing video teleconferencing includes apparatus for compressing video image signals.
Video image signals representative of a live scene at a teleconferencing site usually take an analog form. That is, the continuous physical characteristics of the image, such as brightness and color, are represented by continuous electrical quantities, such as voltage, phase and frequency. However, video image signal compression is generally performed on digital video image signals, due to the ease with which digital signals may be processed by computer systems. Thus, before performing video image signal compression, the analog video image signals are first converted to digital form, wherein the continuous electrical quantities are represented by digital image signals comprising sequences of discrete numbers. Each discrete number is a sample, representative of a value of the corresponding analog signal at a specific point in time. Each specific point in time in turn corresponds to a specific point within the image.
Although the analog video image signal and the digital video image signal are substantially equivalent representations of the physical characteristics of the image, digital video image signals are particularly suitable for processing using computers, as noted above.
Computers can perform a wide variety of arithmetic processes and logical processes on digital signals. Results of operations performed by computers are frequently displayed on a computer display monitor, which resembles a video display. However, it should be noted here that the image signals used to define an image for a television monitor and the signals used to define an image for a computer display monitor differ from each other to a significant extent. A computer display image signal cannot be used directly to display an image on a television monitor; and, a conventional video image signal cannot be used directly to display an image on a conventional computer display monitor. A conversion from one form to the other is required to display an image on a display of one type using a signal of the other type. This conversion is required because, among other differences, the two signals often use different color representations, different vertical resolutions (lines per image), and different horizontal resolutions (picture elements per line).
Thus, conventionally a personal computer (PC) operator wishing to send a computer display image to another site required an arrangement similar to that shown in the block diagram of FIG. 1. In this arrangement, a computer display image in the form of (for example) a video graphics adapter (VGA) signal 101 is output by a VGA card 103 in the PC 105, and is transmitted to a separate device known as a scan converter 107. The scan converter 107, in turn, outputs an analog television signal 109 (e.g., in NTSC or
format, composite or Y/C, etc.), which can be passed to a video compression device 111 or possibly to a video recorder (not shown). The scan converter 107 also outputs another analog VGA signal 113, which is used to drive a computer display monitor 115. Several scan converters are presently on the market, including, for example, the Mediator.TM. product from Video Logic, a UK company.
The above discussion is of general applicability to both monochrome and color images. Modern computer display images are frequently rendered in color, which requires additional apparatus, as now described.
One common way to represent a color computer display image is as three monochromatic image signals, representative of the intensities at each point in the image of the colors red, green and blue. The VGA signal referred to above represents each point in an image as a triad of sample values corresponding to values of the red, green and blue signals at each point in the image. In order to convert the three color digital signals processed by the PC to analog form for input to a computer display monitor, a VGA card in a PC contains three digital-to-analog converters, one each for red, green, and blue signals. The VGA card thus outputs three analog signals, one each for the red, green and blue intensities comprising the image.
A scan converter may contain three analog-to-digital (A/D) converters used to convert the VGA signals to digital form for convenient processing, memory for storing image signals, circuitry for converting the red/green/blue (RGB) color representation of the VGA signal to an interlaced luma/chroma (Y/C) color representation commonly used in television signals, circuitry for converting signals from VGA image resolution to television image resolution, and one or more digital-to-analog (D/A) converters used to convert the output signal from digital form to analog television form. The video compression unit may contain one or more A/D converters necessary to convert and de-interlace the television signal for processing, memory to store the digital television signal during processing, and circuitry to compress and transmit the image. Thus, using this arrangement, a PC display is twice converted to and from analog form.